In cgi/client.py we have:

        # determine if view is oktmpl|errortmpl. If so assign the
        # right one to the view parameter. If we don't have alternate
        # templates, just leave view alone.
        if (view and view.find('|') != -1):
            # we have alternate templates, parse them apart.
            (oktmpl, errortmpl) = view.split("|", 2)

Presumably, the last line should be:

            (oktmpl, errortmpl) = view.split("|", 1)

to avoid a ValueError exception when there is more than one "|".

This can be triggered by something like

  ?%40template=forgotten%7c%7x

as a query parameter.